Nonlinear effects as severe as proof mass impact on mechanical end stops are potentially useful for energy harvesting. In this paper, we present measurements and modeling of a piezoelectric energy harvester with a mechanical end stop on one side. At high acceleration levels we observe nonlinear effects related to impacts on the end stop such as an enlarged up-sweep bandwidth in frequency sweeps. Based on SPICE simulations of an electrical equivalent circuit for the device, we show that modeling of the end stop as a parallel spring-dashpot system is sufficient to recapture the energy harvester behavior.